1. Field of the Invention
This invention relates to I.C. engines, particularly four-stroke I.C. engines employing the Otto or Diesel cycles.
Practical I.C. engines cannot approach the thermal efficiency of the ideal Carnot engine (whose theoretical thermal efficiency is from 77 to 82 percent) due to several well-known factors.
In this invention we have been interested primarily in losses occurring in relation to, particularly inside, the cylinders, and in their effects upon the work done by the engine.
The principal losses are as follows:
a) Losses due to pumping, windage and induction
During the induction stroke in particular (in which the depression arising in the cylinder slows the piston), losses are considerable. These losses increase as engine speed (RPM) increases.
b) Losses due to distribution adjustment or exhaust
It is known that scavenging of combustion gases in the exhaust stroke involves overcoming a greater than atmospheric pressure, even with the exhaust valve fully open. This prevents complete expulsion of the combustion gases from the cylinder and causes a corresponding decrease in performance.
c) Cooling losses
These quantitatively important losses are intentionally caused so that the engine is kept within an optimal temperature range dictated by material constraints. Most of this energy is dissipated by the engine's cooling system.
By way of background to the invention, reference will now be made to FIGS. 1 to 7 of the accompanying drawings, which summarize some of the history of I.C. engine technology.
Initially, engines were built with a long stroke and with only one large cylinder and piston, with the purpose of maximum use of gas pressure. These engines were economic but their rotational speed was limited by the large moving masses.
The prevailing aim initially was to maintain valve timing as near as possible to the ideal cycle; however, this led to poor power to weight ratios.
In time, development led to the multiplication of cylinders to allow higher RPM, and therefore more power, by decreasing the size of the moving masses.
In an early vehicle engine, shown in FIG. 1, the valves (a) are located beside the cylinder (b) and are directly moved by a camshaft (c) located near the crankshaft. The combustion chamber (d) situated over the valve heads (e) is offset with respect to the cylinder. The very sharp curves of the gas ducts (f) communicating with the combustion chamber slow the intake and exhaust gas flows. Moreover, the flame front, during expansion, is not centered over the piston (g). For these reasons the engine suffers from low efficiency. Nevertheless, this type of engine is still used in building heavy work machines, due to its economic manufacture.
In later engines, as shown in FIG. 2, performance was improved by aligning the combustion chamber (a) with the piston head (b), which requires that the valves (c) are inverted and situated in the cylinder head (d); the so-called overhead valve (OHV) arrangement. This requires a more complex push rod mechanism (e) to operate the valves. The combustion chamber has to be smaller to obtain higher compression ratios, which reduces the size of the valves (c), but the advantage is that the gas ducts (f) are straighter and so gas flow is improved, obtaining a remarkable performance increase.
As even better performance was sought, the compression ratio was increased still further. There was also a need to increase valve diameter, which led to the development of a hemispherical or `pent-roof` combustion chamber, as shown in FIG. 3, in which inlet and exhaust valves (b and c) are inclined. This allows the inlet and exhaust ducts (d and e) to be straighter and to have an increased cross-sectional area. Additionally, the flame front is centered with respect to the piston by virtue of the central location of the spark plug (L).
In the construction illustrated in FIG. 3, camshafts (f and g) are situated in or over the cylinder head (h) to act directly upon the valves. Engines of this overhead camshaft (OHC) type are now in widespread use, particularly in sports cars and motorcycles. Other engines have the camshaft below the valves but with the valve actuation mechanism arranged in such a way that the valves can operate at an angle.
Another system used in some engines, as shown in FIG. 4, has a valve (a) in the cylinder head (b) and another (c) in the block (d). This partly solves the problems outlined above. These engines, designed with a long stroke, are quite efficient, have a satisfactory RPM limit, and are mostly used in alternative vehicles.
At the same time as progress was being made on the cylinder head, the piston stroke was modified step-by-step as engines became more efficient. The first engine designs and constructions almost always had a long stroke (FIG. 5), i.e., were substantially `undersquare` but afterwards the stroke began to shorten. Thus, less gas entered and left the cylinder per cycle, allowing RPM to be increased. In due course, engines approached a medium stroke (FIG. 6) as the stroke approached the bore. This approximately `square` configuration, adopted by most current engines, allows satisfactory performance, a clean exhaust and substantially complete combustion. Then, however, the need for smaller, more powerful engines initiated a series of short-stroke `oversquare` engines (FIG. 7) that reduced the moving mass even more (allowing increased RPM) but as a negative side effect, gave poor emissions performance.
The valve distribution diagrams for each stroke arrangement are shown at the feet of FIGS. 5, 6 and 7. It can be seen that valve overlap is always symmetrical and that it increases as stroke decreases.
The next efforts to improve efficiency were centered upon, for example, the addition of fuel injection, increasing the number of valves per cylinder, and forced induction, whether by turbocharger or supercharger. Not many attempts, however, were made to redesign the engine's basic structure.
In summary, it can be pointed out that the most satisfactory engine structure is the one depicted in FIG. 3 having the strokes of FIGS. 6 or 7.
Until now, many problems remain unsolved. For instance:
a) heat removal;
b) the sensitivity of materials to high temperatures;
c) structural complexity, leading to difficulty in manufacture;
d) weight reduction with respective to effective power; and
e) optimizing thermal efficiency.